Ganymede (Greek Γανυμήδης or Ganumedes) or Jupiter III is the largest of the Galilean moons of Jupiter and indeed the largest planetary satellite, by mass or radius, in the solar system. It is slightly larger than Mercury (though Mercury is twice as heavy) and is both larger and heavier than any of the dwarf planets Eris, Pluto, and Ceres. Thus, if it orbited the Sun instead of Jupiter, it might be a true planet—if it "cleared its neighborhood" of all other objects, as all planets must do to be worthy of the name.
Discovery and naming
Galileo Galilei observed Jupiter beginning on January 7, 1610, with his famous telescope. He at first thought he had discovered three stars near Jupiter, but on the next night those "stars" seemed to have moved. He continued to observe Jupiter and its companion "stars" for seven days, during which time a fourth "star" (actually Ganymede) appeared and all four of these objects appeared to move with Jupiter. Finally he concluded that these objects were not stars at all, but satellites of Jupiter. This was the first direct observation that provided evidence for Nicolaus Copernicus's heliocentric model of the solar system.
discoverer::Simon Marius claimed to observe Jupiter and these satellites independently of Galileo and beginning five weeks earlier. However, he did not publish his findings, while Galileo published his. Furthermore, Galileo's notes were more reliable and extensive than those of Marius, which is why Ganymede and the three other satellites he observed (Io, Europa, and Callisto) are called the Galilean moons and not the Galilean-Marian moons.
Marius does, however, receive credit for providing the names that the satellites have today. He named Ganymede, the largest, for the mythical son of King Tros of Troy, carried aloft to Mount Olympus by Zeus to be the cupbearer to the gods. (The names Zeus and Jupiter refer to the same classical deity from whom Jupiter gets its name.) The other three satellites are named for three of Zeus' most famous mistresses. Marius propounded these names after Johannes Kepler suggested them to him.
Galileo, for his part, called them the "Medicean planets" after the Medici family and simply numbered the moons I, II, III, and IV. The satellites carried these names for two centuries until the discovery of other moons of Jupiter made that naming system untenably confusing.
Orbital and rotational characteristicstidal lock (thus keeping the same face toward Jupiter as it revolves), its axis does tend to tilt very slightly from the true perpendicular from time to time on account of gravitational perturbations.
The most striking feature of Ganymede's orbit is its three-part orbital resonance or Laplace resonance with the two inner moons Io and Europa. These three satellites complete their orbits around Jupiter in the ratio 1:2:4. They also conjoin with one another at their apsides, in a mutually self-correcting manner that allows the resonance to persist. (Most such resonances are unstable and end with one or all bodies changing their orbits.)
Yet this resonance alone cannot account for Ganymede's eccentricity today. Neither, as Showman and Malhotra have recently shown, can an asteroid or cometary impact account for the eccentricity presently observed.
The origin of the resonance itself is controversial. Peale and Lee, in 2002, presented a model suggesting that this resonance is primordial and thus was part of the Jovian system since its formation. Their model, however, derives from the nebular hypothesis of the formation of the solar system and thus depends on uniformitarian assumptions. Showman and Malhotra suggest a model by which the resonance developed after the solar system had formed.
Physical characteristicsMercury and more massive by an order of magnitude than either Eris or Pluto. Its density is 1,942 kg/m³, which is consistent with a composition that is half water (or water ice) and half rock. Much of the surface is covered with water ice, which produces the light-colored terrain in most true-color images of this moon.
The model of Ganymede's interior has its basis in this most striking finding: Ganymede and Io, unique among all planetary satellites, have their own magnetic fields. This is most consistent with Ganymede having a metallic core, most likely of iron and iron sulfate. Surrounding this core is a mantle of ice and silicates and an outer mantle of ice.
Ganymede appears to have a strong inductive magnetic moment. Its permanent equatorial magnetic flux density is 7.19 milligauss, stronger than that measured at any other moon. This is consistent with a magnetic dipole moment of 1.31 * 1020 N-m/T. The magnetic dipole moment at creation was probably 3.49 * 1022 N-m/T, consistent with a decay time of about 1099 years and a half-life of 762 years.
However, the permanent magnetic moment does not account for all the magnetic fluxes on Ganymede. Ganymede lies well within Jupiter's magnetic field, and this field is likely inducing an electrical current within a conducting shell having a radius almost as long as that of Ganymede itself. This shell is probably a conductive liquid between the crust and the core, most likely a saltwater ocean about 146 km deep to the surface and extending to the outer mantle.
In 1995, the Hubble Space Telescope team developed evidence for the manufacture of ozone on Ganymede. Speculation on the origin of this ozone has centered on radioactive particles trapped within Jupiter's own magnetic field in a Van Allen-type radiation belt that actually extends beyond Ganymede's orbit.
Problems for Uniformitarian Theory Posed by Ganymede
Ganymede is supposed to be a "geologically old" body because it is heavily cratered and because "new" terrain appears to overlay "old" terrain. Furthermore, at such a vast distance from the Sun (5.2 AU), the metal core ought to have cooled sufficiently for the magnetic field to die out. Yet the field persists. Astronomers have attempted to explain the persistence of the magnetic field by invoking tidal heating. However, the Russell Humphreys model explains the field quite well. According to Humphreys, every celestial body begins with a magnetic field. The volume and conductivity of a celestial body's core, and not its temperature, determine the decay time of that field.
The 4:2:1 resonance among Ganymede, Europa, and Io presents another problem: the orbit of Ganymede ought not be quite as eccentric as it is. Explanations for this have not been universally satisfactory.
Observation and Exploration
Observation of Ganymede begins, obviously, with its discovery by Galileo. The Hubble Space Telescope has also been used to observe Ganymede.
The first space-borne exploration of Ganymede was by Visiting mission::Pioneer 10 and Visiting mission::Pioneer 11, but the information they gleaned was minimal. Visiting mission::Voyager 1 and Visiting mission::Voyager 2 provided much more information, including a clarification of Ganymede's size and mass and the establishment of Ganymede, and not Titan, as the largest moon in the solar system.
By far the most comprehensive exploration to date was carried out by the Visiting mission::Galileo project and its spacecraft, that flew by Ganymede several times in its study of Jupiter and the Jovian system.
A further exploratory venture, the Jupiter Icy Moons Orbiter, was cancelled in 2005. At the time of its cancellation, many scientists had criticized the program as an unrealistic venture.
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